Apparatus and methods for determining the minority carrier diffusion length (L) using the surface photovoltage method are well known. In brief, the principle of the diffusion length (L) determination requires the illumination of a specimen surface with monochromatic radiation of energy slightly greater than the bandgap of the semiconductor. Electron-hole pairs are produced and diffuse to the illuminated (front) surface where they are separated by the electric field of the depletion region (i.e., the surface-space-charge region) to produce a surface photovoltage. A portion of the surface photovoltage signal is coupled to an amplifier for amplification and measurement. The photon flux (photons per sq. cm. per second) is adjusted to produce the same magnitude of surface photovoltage at various wavelengths of illumination. The photon flux required to produce this constant magnitude surface photovoltage signal is conveniently plotted on the ordinate against the reciprocal of the absorption coefficient on the abscissa for each wavelength. The resultant plot is typically linear and is extrapolated to the zero photon flux intercept on the negative abscissa. This intercept value is the effective diffusion length (L). For a more detailed description of the theory and background for this method, see an article "A Method for the Measurement of Short Minority Carrier Diffusion Lengths in Semiconductors," by A. M. Goodman in the Journal of Applied Physics, Vol. 32, No. 23, pp. 2550-2552, December 1961 and the article by A. M. Goodman entitled "Improvements In Method and Apparatus For Determining Minority Carrier Diffusion Length", International Electron Devices Meeting, December 1980, pp 231-234. The American Society for Testing and Materials has adopted a standard using this method which is published as ASTM F 391-78. The ASTM standard, when implemented according to the block diagram of FIG. 1 of ASTM F 391-78, is provided particularly for testing the diffusion length (L) for minority carriers in silicon but the method in general may be used for other semiconductor materials.
See U.S. Pat. No. 4,333,051, incorporated herein by reference thereto, entitled "METHOD AND APPARATUS FOR DETERMINING MINORITY CARRIER DIFFUSION LENGTH IN SEMICONDUCTORS", issued on June 1, 1982 to A. M. Goodman for a description of an apparatus using this principle in which a servo system maintains a constant predetermined value of the surface photovoltage thereby allowing the measurements to be carrier out in a relatively short time. The surface photovoltage pickup electrode described in this patent minimizes the effects of drift caused by laterally diffusing minority carriers during a test.
In using the method F 391-78 of the American Society for Testing Materials, it is important to treat the surface of the material being tested in a way that provides for consistently large and stable surface photovoltage. The standard F 391-78 provides for certain conditions prior to the surface photovoltage testing including cleansing the surface and etching the surface if necessary. It has been determined by many uses of this method that the surface treatment recommended by the Standard F 391-78 for n-type Si wafers has been found to be unreliable. Moreover the results obtained from the use of the ASTM method depend strongly upon the sample's previous history. For example, as a silicon wafer is withdrawn from a hydrofluoride (HF) etching solution into the air, a stain film may be formed on the silicon surface. The stain film has a deleterious effect on the surface photovoltage signal in that is is either too low or unstable, or both. As shown in the art such a stain film can be avoided by quenching the HF-containing etch with distilled water before withdrawing the wafer into the air. Other techniques are known for obviating the effect of the stain film.
Nevertheless, even with great care in avoiding the stain film, the preparation of the surface of the material according to the technique suggested in the standard F 391-78, note 4, of boiling the material for n-type silicon, resulted in the surface photovoltage being still erratic and too small. Moreover, even when the surface photovoltages were large enough, the surface photovoltage tended to change with time. Such a condition hereinafter referred to as instability can cause a significant error in the measurement result. In one example, a 3% change in the photon flux (I.sub.o) being used to maintain the surface photovoltage to a constant value during a 3-minute time period of measurement resulted in an inaccuracy of approximately 10% in the diffusion length determined. Errors in diffusion length (L) due to this instability are undesirable and errors larger than about 10% are to be avoided.
Accordingly, there is a need in the art for a surface preparation method to provide surface photovoltages that are both sufficient and stable for determining minority diffusion length (L).